1. Field of the Invention
The invention relates to apparatus for detecting grounds in high voltage static converters in general, and more particularly to apparatus including a circuit arrangement for detecting grounds in a high-voltage static converter having three a-c leads, wherein the circuit arrangement comprises: three electrical components each having first and second leads or terminals, the first leads of these components being connected to the a-c leads of the converter, respectively, and the second leads being connected together to form an artificial neutral point; and a current-measuring circuit means connected between the artificial neutral point and ground for delivering an alarm signal in the event of a ground of the converter.
2. Description of the Prior Art
In the operation of an electrical system, it is frequently necessary to quickly and reliably detect and indicate the occurrence in the system of a short circuit to ground. This permits the sounding of a trouble alarm and/or the taking of other protective measures without any significant delay. Such a short circuit to ground may be due to a direct low-resistance connection ("full ground") between a point of the system and ground or to a connection with resistance between such point and ground. The latter type connections occur, for example, when the insulation of the system is inadequate, thereby promoting the development of leakage currents or arcs.
Siemens-Zeitschrift 46 (1972), no. 12, pages 909 to 911, particularly FIG. 1, discloses an apparatus which includes a circuit arrangement for detecting grounds in the stator winding of a high-voltage electric machine. This circuit arrangement comprises, amongst other components, a grounding transformer having a primary winding formed of three windings each having fist and second ends. The first ends of these windings are connected to the respective three phase conductors of the machine and the second ends of the windings are connected together to form an artificial neutral point which is directly connected to ground. The grounding transformer has a secondary winding which also includes three windings, these windings being connected together to form an open triangle or delta circuit. A supplementary 20 Hz voltage is fed to the delta circuit so that the three-phase system is maintained at a potential above ground. An alarm signal for indicating the occurrence of a ground is derived from a further transformer which is arranged on the secondary side of the grounding transformer.
Such a circuit arrangement cannot, however, be used for monitoring grounds in an a-c voltage system which is metallically connected (d-c coupled) to a static converter. This is so because the displacement voltage arising during a short to ground in such a system can include a d-c component. A d-c component always occurs when the d-c side of the converter is grounded, i.e., when the plus or minus lead of the converter is connected to ground via a larger or smaller resistance. The latter grounding can occur, for example, when due to defective insulation, a short occurs between the primary and secondary winding of the trigger transformer of one of the controlled electric valves of the converter. In any case, the occurrence of a d-c component in the displacement voltage would cause a large current to flow in the primary winding of the grounding transformer of the aforesaid ground detecting circuit arrangement and would thereby lead to malfunctioning or even to the total destruction of the transformer.
The text "Fehler and Fehlerschutz in elektrischen Drehstromanlagen" (Faults and fault protection in electrical three-phase systems) by H. Titze, vol. 2, Springer-Verlag, Vienna 1953, page 68 discloses a further apparatus including a circuit arrangement for detecting grounds. In this circuit arrangement, three voltage transformers are employed. The first ends of the primary windings of these voltage transformers are connected respectively to three a-c conductors and the second ends are connected together to form a common artificial neutral point. This artificial neutral point is again connected directly to ground. In this circuit arrangement, the three secondary windings of the transformers are delta connected and a voltage is derived therefrom which corresponds to three times the displacement voltage.
Again with this circuit arrangement, the occurrence of a d-c component in the displacement voltage can lead to grave malfunctions. Thus, such a d-c component, even if of a relatively small value, can cause the voltage transformers to go into saturation. This circuit arrangement, therefore, also cannot be used for detecting grounds in a static converter.
Siemens Brochure LE 2, 1975, pages 6/65 to 6/68, Section "Elektronischer Erdschlusswaechter EEW 1" (Electronic Ground Monitor EEW 1) discloses a further apparatus which includes a circuit arrangement for detecting grounding of a static converter via the converter insulation. This circuit arrangement contains three high-resistance, purely ohmic resistors, whose first ends are connected to the a-c leads of the converter. The second ends of the resistors, in turn, are connected together to jointly form an artificial neutral point. An auxiliary winding of a current detection circuit including a toroidal-core transformer is connected between this neutral point and ground. In undisturbed operation, the measured displacement voltage is zero, and no ground current flows through the auxiliary winding of the detection circuit. In the event of a ground, however, a finite resistance to ground results between the three-phase network connected to the converter and ground. This causes a certain amount of asymmetry of the phase voltage relative to ground and an appreciable ground current now flows through the auxiliary winding. This ground current flow is sensed by the toroidal-core transformer, which in response thereto develops an alarm signal. After further transformation and amplification, the latter alarm signal is used to activate a disconnect relay.
In the last discussed circuit arrangement, the occurrence of a d-c component in the displacement voltage will not jeopardize operation of the circuit. However, the above-mentioned resistors of this arrangement are always under load i.e., are under load not only in the event of a ground, but also during normal operation of the converter. The three resistors must, thus, be designed not only for high voltage but also for continuous operation. As a result, the resistors are difficult to obtain and very costly. Moreover, such resistors occupy a great deal of space, so that they necessitate a considerably large volume in the circuit arrangement. Also, these resistors are disadvantageous due to their continuous power loss and the heat they produce.
It is, therefore, an object of the present invention to provide a circuit arrangement for detecting grounds in a static converter which uses only a small amount of power during the normal operation of the converter and which is additionally capable in the event of a ground of handling a displacement voltage containing a d-c component.